The present invention is in the field of battery technology and, more particularly, in the area of improved active materials for use in electrodes in electrochemical cells.
Research into active materials for cathodes for secondary batteries has yielded several classes of active materials. One class of active materials is a type of “over-lithiated” layered oxide (OLO) represented as:xLi2MnO3·(1−x)Li[MniNijCok]O2 where 0≦x≦1, i+j+k=1, and i is non-zero. Such OLO materials are promising candidates for next generation batteries because of their high specific capacity.
However, OLO materials suffer from a large irreversible capacity loss during the first cycle of use in an electrochemical cell. Batteries fabricated with OLO materials are assembled using a non-activated form of the OLO material. On first cycle, the non-activated material is electrochemically activated by simultaneous extraction of lithium in the form of Li+ and oxygen in the form of O2 or other oxygen-containing gasses. This activation process has several drawbacks. First, gas is generated, which can lead to problems in cell manufacture. Second, defects may be generated in the surface and bulk, which can reduce rate capability, increase the rate of metal dissolution, and increase the rate of electrolyte oxidation. Third, the extracted lithium may form unstable lithium species on the anode that interfere with typical anode stabilization.
OLO has been chemically activated by reaction with aqueous acids, such as hydrochloric acid (HCl). Such an activation process has several drawbacks. First, this activation process requires an excess of acid, which can make it difficult to control the extent of lithium extraction such that an insufficient amount or excess amount of lithium is extracted. Second, water and/or protons can become incorporated into any vacancies generated by the activation and this can lead to poor cycle life and rate performance. Third, the disposal of the wastewater from this process can be costly due to the chemical contaminants in it, making the process difficult to scale up.
Some research has been conducted into the use of organo-fluorides, such as polyvinylidenefluoride (PVdF) and polytetrafluoroethylene (PTFE), to remove alkali-ions from metal oxides. In some research, the organo-fluoride was used to remove both alkali ions and oxygen. See, e.g., T. Ozawa et al., Inorg. Chem, 49, (2010) 3044 and T. Ozawa et al., Inorg. Chem, 51 (11), (2012). The reaction was used to remove all of the alkali-ion from the material.
Further, there has been some work on the use of aluminum fluoride (AlF3) coatings to improve the electrochemical performance of lithium-rich layered oxides. See, e.g., Scrosati, B. et al., Adv. Mater. 2012, 24, 1192-1196 and Zheng, J. M. et al., J. Electrochem. Soc. 2008, 155 (10), A775-A782. Still further, U.S. Patent Publication 2013/0216701 discloses an AlF3 coating formed from precursors including an aluminum nitrate solution and an ammonium fluoride solution. No AlF3 was used directly in the coating process. There is no evidence in the publication to suggest any chemical reaction between the OLO and the metal fluorides and no evidence of unit cell changes in the OLO structure. The coating process must take place in nitrogen or other oxygen-free atmosphere. And still further, International Publication WO 2006/109930 discloses the use of elemental precursors such as metal alkoxide, sulfate, nitrate, actetate, chloride, or phosophate to coat lithium metal oxide materials.
Some research has been conducted on delithiating LiNiO2 for rechargeable batteries. See, e.g., Arai, H. et al., Electrochem. Acta 2002, 47, 2697 (for use of sulfuric acid) and Arai, H. et al., J. Power Sources 1999, 81-82, 401 (for use of NOPF6). In the work using NOPF6, the PF6 reacts with the LiNiO2 and generates an LiPF6 salt and NO gas. In this reaction, all the fluorine atoms remain bonded to the phosphorous. Further, the degree of delithiation was low in spite of the excess amount of NOPF6 (NOPF6/LiNiO2=2).
There remains a need for an efficient, scalable means of pre-activating OLO active materials through controlled lithium extraction for the fabrication of full cells for use as primary and secondary batteries.